Dye sensitization of electrophotographic materials



Sep-t' 4, 1962 H. G. GREIG 3,052,540

DYE SENSITIZATION OF' ELECTROPHOTOGRPHIC MATERIALS Filed April 20, 1959 2 Sheets-Shea?. 2

a 4 f n ljjn f F #f V J INVENTOR. Meow 6 Gef/6 United States Patent G 3,052,540 DYE SENSITIZATION F ELECTROPHOT0- GRAPHIC MATERIALS Harold G. Greig, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed Apr. 20, 1959, Ser. No. 807,714 14 Claims. (Cl. 96-1) This is a continuation-impart of U.S. patent application Serial No. 433,898, filed June 2, 1954, and now abandoned, by H. G. Greig, the applicant herein, and assigned to the same yassignee thereof.

This invention relates to photoconduotive materials and devices particularly useful in electrostatic printing and to methods of making and using said photoconductive devices.

An electrostatic printing process is that type of process for producing a -visible record, reproduction or copy which includes as an intermediate step, converting a light image or electrical signal into an electrostatic charge pattern on an electrically-insulating ibase. The process may also include the conversion of the charge pattern into a visible image which may be a substantially faithful reproduction of an original except that it may be a different size.

A typical electrostatic printing process may include coating -a surface of a relatively conductive backing member with a photoconductive insulating material such as selenium, anthracene, or sulphur, and then providing an electrostatic charge on the surface of the photoconductive coating. A light image is focused on the charged surface, discharging the irradiated areas, While leaving the remainder of the surface in a charged condition, thereby forming an electrostatic image or pattern of electrostatic charges. The electrostatic image may be rendered visible by applying thereto developer powder particles which are held electrostatically to charged areas of the sheet. The powder image thus formed is transferred to another surface upon which the reproduced image is desired and then xed thereon. A detailed description of the steps of such a process may be found in U.S. Patent 2,297,691, issued October 6, 1942, to C. F. Carlson.

An improved electrostatic printing process is described by H. G. Greig in U.S. patent application, Serial No. 383,677, filed'October l, 1953, and in C. l. Young and H. G. Greig, ElectrofaX-Direct Electrophotographic Printing on Paper, RCA Review, volume 15, No. 4, pages 469 to 484 (December 1954). Greigs improved process eliminates the intermediate step of transferring the powder image and produces the powder image directly upon the desired surface. A recording element for his improved process comprises a backing sheet, such as paper, coated with specially-selected photoconductive white zinc oxide particles suspended in 'an electricallyinsulating film-forming binder.

The photoconductive white zinc oxide-binder recording elements have been found to exhibit unusual and nnexpected characteristics especially useful in electrostatic printing. The spectral response thereof peaks sharply in the near ultraviolet region of the spectrum with sensitivity extending into the blue end of the visible region of the spectrum. This spectral response characteristic is advantageous for many applications. However, in other lCC applications -a broadened spectral response is desirable in order to more closely match the spectrum of the light image to which the recording element is exposed or to provide color separation of the light image.

It is an object of this invention to provide improved photoconductive materials and devices having an extended and predetermined spectral response especially useful as recording elements for electrostatic printing.

Another object is to provide improved methods of and means for extending the spectral response of photoconduc-tive white zinc oxide-binder recording elements for electrostatic printing.

Another object is to provide methods of and means for adsorbing an optically sensitizing dye to photoconductive ywhite zinc oxide particles in recording elements for electrostatic printing.

Another object is to provide improved methods of and means for electrostatic printing.

In general, the invention includes a photoconductive insulating material comprising finely-divided photoconductive white zinc oxide particles in an electrically-insulating, film-forming binder, and having incorporated therein yan organic dye which is capable of absorbing radiant energy in a -band of wave lengths to which said zinc oxide particles are substantially insensitive and transferring said adsorbed energy to said zinc oxide particles. A recording element for electrostaticprinting may comprise -a backing member having a sur-face coated with a photoconducting insulating material of the invention.

The photoconductive insulating materials herein are characterized by exhibiting the photoconductive spectral response of white zinc oxide-binder recording elements and al-so an additional photoconductive spectral response in the visible region of the spectrum. The additional spectral response is definite `and predetermined, being a characteristic of the particular organic dye used to sensitize the recording element. Since many of the organic `dyes used herein lfor extending the spectral response may be bleached easily and conveniently, it is possible to combine the advantages of wider spectral response with a white background where this is desirable.

The organic dye used herein for extending the spectral response of a recording element herein is `adsorbed upon the surface of the zinc oxide particles. This adsorption may be achieved by staining the white zinc oxide particle with the dye before mixing with the binder, byadding the dye to the zinc oxide-binder coating dispersion before coating the mixture upon a support, or by staining the completed coating with the dye.

The invention includes also methods for using the recording elements herein in an electrostatic printing Iprocess. In order to remove the color of the dye, the dye may be bleached to a colorless material after the powder image has been for-med. It has been found unexpectedly that one convenient method yfor bleaching is simply to expose the recording element to light which by itself, bleaches the dye in many of the recording elements herein.

The foregoing objects and other .advantages are described in greater detail in the following `description by reference to the accompanying drawings in which:

FGURE l is a family of curves illustrating the eifect of dye concentration upon the spectral sensitivity of a recording element of the invention;

FIGURE 2 is a family o-f curves illustrating the effect of different sensitizing dyes upon the spectral sensitivity of recording elements of the invention;

FIGURE 3 is a partially schematic sectional view of an apparatus for producing a blanket electrostatic charge upon the surface of a recording element of the invention;

FIGURE 4 is an elevational view of an apparatus for projecting an optical image upon the charged surface of the recording element of FIGURE 3; and

FIGURE 5 is a sectional View of an apparatus for developing an electrostatic image upon the recording element of FIGURE 4.

Similar `reference characters are applied to similar elements throughout the drawing.

EXAMPLE 1 An improved electrophotographic recording element may be prepared bycoating a paper backing member with a photoconductive insulating material of the invention. To prepare a preferred photoconductive insulating material, mix the following ingredients:

80 grams of a silicone resin solution (containing 60% f solids), such as GE Silicone Resin SR-SZ, marketed by "YfPrepare a solutionofv-04 gram of Rose Bengal dye vdissolved in cc. of methyl alcohol or other suitable solvent. This corresponds to about 0.033 weight percent dye with respectto the weight of zinc oxide. A suitable dye is Rose Bengal, C.I. No. 779 (First Edition), manufactured by the National Aniline Division, Allied-Chemical and Dye Corp., New York, N.Y. Thisdye is also 'identified as C.I. 45400, C.I. Acid Red 94 (Second Edition), and as Tetraiodinate 4,5,7-tetrachloro iluorescein. This solutionis added slowly to the zinc oxide mix with constant stiiring. The mix is then ball-milled in a porcelain mill with porcelain balls for about three hours to insure complete mixing of the ingredients and to obtain a smooth, uniform consistency for the subsequent coating step.. Y

- The milledmix is then coated upon a sheet of paper and dried to remove the solvent. The dried coating layer is about .0001 to .0015 inch thick but preferably about .0005 inch thick for most applications.

In FIGURE 1curve B represents the relative spectral response of the recording element of Example l. Curve A illustrates Ythe spectral response of a similar recording element containing about half the amount of dye (about 0.0167 weight percent) contained in the element of curve B. Curve C illustrates the spectral response for a similar recording element containing about twice the amount of dye (about 0.067 weight percent) contained in the element of curve B. It will be noted that the curves peak at about375'0 A. and between 5250 and 5750 A. When no dye is added, the curve has only one peak at about 3750 A. Y

FIGURE 2 shows a set of comparative curves wherein the dye (Rose Bengal) of Example l is replaced with other dyes such as eosin Y, iiuorcscein, erythrosin and crystal violet. When no dye is added, the element exhibits a single peak at about 3750 A. Where a dye has been incorporated, each element exhibits two peaks, one at about 3750 A. and one in another area of the spectrum and generally adding sensitivity in a band of wavelengths to which zinc oxide particles are substantially insensitive.

The curves of FIGURES l and 2 are obtained in the following manner. A recording element is prepared according to Example l. The surface of the coating is then electrostatically charged to 600 volts, for example with a corona discharge apparatus. Incremental areas of the surface are exposed to light of a very narrow band of wavelengths. A standard Bausch and Lomb monochrometer having a tungsten lamp light source may be used to obtain the desired band of wavelengths. The curves of FIGURE 1 were obtained with a 0.5 second exposure and the curves of FIGURE 2 were obtained with a. 1.0 second exposure. The voltage of the surface of the coating with respect to ground is measured before and after the exposure by means of a vibrating probe electrometer. The curves of FIGURES l and 2 represent a compilation of the voltage `drops obtained by exposure to the respective wavelength bands for each combination of dye and zinc oxide-resin mix.

While a specific example of a recording element has been given, other materials and structures may be used. The backing member may be either a relatively insulating material such as paper, or it may be a relatively conducting material, such as metal foil or sheet, or paper loaded with carbon black.

In the research work to develop the electrophotographic recording elements described in patent application Serial No. 383,677, op. cit., it lwas noted that many white zinc oxides tested produced recording elements which did not print or printed very poorly. These white zinc oxides are referred t0 as semiconductive white zinc oxides. Most of these'semiconductive 'white zinc oxides were made by the American process directly from Zinc ore. Measurements of the equivalent voltage of the surface immediately after electrostatic charging in the dark indicated little or no retention of surface electrostatic charge. This is believed to be due to the rapid decay of electrostatic charge on coatings made with the semiconductive zinc oxides. i

The remaining white zinc oxides, referred to as photoconductiveV white zinc oxides, produce coatings which retain sufficient electrostatic charge after electrostatic chargingand may be used in the recording elements herein for electrostatic printing processes.

By special test disclosed in patent application Serial No. 383,677-, op. cit., the semiconductive and photoconductive white zinc oxides may be distinguished by value of surface photoconductivity. The photoconducting white zinc oxides all have a valuevof surface photoconductivity higher than l0*9 ohms-"l/square/watt/cm; whereas the semiconducting white zinc oxides all have values below this value.

The electrically-insulating, film-forming binder (also referredto as vehicle) may be selected from a large group of substances. It is desirable for the binder to have a `relatively high dielectric strength. These materials may be natural or synthetic resins or waxes. Examples of suitable resins are the Vinyl resins, silicone resins, phenol formaldehyde compounds and cellulose ethers and cellulose esters. Shellac is an example of a suitable natural resin. Examples of suitable waxes are parain, carnauba wax and beeswax. Mixtures of two or more binders may be used. Plasticizers or similar modifying agents may be incorporated with the llmforming binder provided they do not adversely affect the electrical properties of the material.

The following dyes in the Table of Dyes, representing several different dye classes, are examples of dyes which sensitize the photoconductive white zinc oxide-binder recording element of Example l. By sensitize is meant capable of absorbing radiant energy in a band of wavelengths to which Zinc oxide particles are substantially insensitive and transferring said absorbed energy to the photoconductive white zinc oxide particles to produce a change in the conductivity of said zinc oxide particles. In each case, the recording element exhibits a peak at about 3750 A. in the original range of the white zinc oxide. However,V in each case the recording element exhibits an additional peak over a narrower or wider range at some other place on the spectrum. C.I. denotes Color Index number (Second Edition, 1957).

Table of Dyes O.I.=Color Index No. (2nd Ed., 1957) I. Xanthene Dyes:

45350 (0.1. Acid Yellow 73). 45380 (C. Acid Red 87). 45405 (C. Acid Red 98). 45410 (C. Acid Red 92). 45430 (C. Acid Red 51). Rose B engel 45440 (C. Acid Red 94).

II. Triarylmethaue Dyes:

Malachite Green Basic Green 4).

I I I I I Basic Violet 3).

I I I Crystal Violet. n

Basic Fuchsine Basic Violet 14). Methyl Green l Brilliant Gree Basic Green l).

. Acid Blue 1). Acid Blue 9).

. Basic Yellow l).

Patent Blue Brilliant Blue FGF III. Thiazole Dyes:

Thioavine TG 1V. Thiazine Dyes:

Methylene Green Methylene Blue V. Aziue Dyes:

. Basic Green 5), Basic Blue 9).

Neutral Red 50040 (C.I. Basic Red 5).

Safranine Y 50240 (Cl. Basic Red 2).

Methylcne Violet 50205 (0.1. Basic Violet 5). VI. Acridine Dyes:

Acridne Orange 46005 ((3.1. Basic Orange 14).

VII. Diphenylmethane Dyes:

Auramine O VIII. Cyanine Dyes:

41000 (Cl. Basic Yellow 2).

Ethyl Red (Absorption Max. in alcohol 5002 A.)

Pinacyanol..

Thiazolc Purple IX. Mixture oi Dyes:

Methylene Grey X. Anthraquinone Dyes:

A lizarine Red (asf-diamants earbocyanme idide.)

50431 (0.1. Basic Black l).

58005 (Cl. Mordant Red 3).

Quinizarin 58050 (C.I. Pigment Violet 12). Carbanthrene Yellow G (Flavauthrone) 70600 (C.I. Vat Yellow 1),

80 grams silicone resin solution SR-82 (60% solids in xylene-mfg G.E.) 106 grams toluene 120 grams zinc oxide, Florence Green Seal-8 (mfg. New

Jersey Zinc Co.)

EXAMPLE 2 The addition of the following dye mixture to the base coating mix produces recording elements having a panchromatic response balanced for approximately equal sensitivity to blue, green and red light from a 100 watt incandescent lamp.

0.012 gram Fluorescein (sodium salt) .005 gram Rose Bengal (potassium salt 88%) .020 gram Brilliant Green (nat. aniline 93%) m1. methanol The finished coating is slightly olf white in color. For increased sensitivity with a darker (gray-green color) colored coating the dyes can be increased in ratio.

EXAMPLE 3 The addition of the following dye mixture to the base coating mix produces recording elements having a panchromatic response but with widely different sensitivities to red, green and blue light for exposure to a 100 watt incandescent lamp as the light source.

0.0063 gram Patent Blue, Cl. 42045 .0025 gram Rose Bengal (88%), Cl. 45440 5 m1. methanol The finished coating is slightly off-white having a greenish cast. Here again the overall sensitivity can be increased by increasing the dyes in ratio.

B EXAMPLE 4 0.005 gram Rose Bengal (88%), C.I. 45440 .020 gram Brilliant Green, Cl. 42040 .040 gram Thioflavin TG, C.I. 49005 5 ml. methanol The finished coating has a slightly greenish cast.

EXAMPLE 5 The addition of the following dye mixture to the base coating mix produces recording elements having a panc'hromatic response with slightly higher sensitivity to red and blue light than it does to the green light from a watt incandescent lamp.

0.015 gram Fluorescein (sod. salt), C.I. 45350 0.015 gram Patent Blue, C.I. 42045 5 ml. methanol The finished coating has a slightly gray-green cast.

EXAMPLE 6 The addition of the following dye mixture to the base coating mix produces recording elements having a panchromatic response but exhibiting widely different sensitivities to the red, green and blue light from a 100 Watt incandescent lamp.

0.060 gram Brilliant Blue FCF (Cl. 42090) 0.036 gram Rose Bengal (C.I. 45440) 5 ml. methanol The dried coating has a blue color.

EXAMPLE 7 The addition of the following dye mixture to the base coating mix produces a light gray coating that exhibits a well balanced response to the red, green and blue light from a 100 watt incandescent lamp.

0.020 gram Rose Bengal (C.I. 45440) .i010 gram Fluorescein (sod. salt) (C.I. 45350) .015 gram Auramine O (C.I. 41000) .005 gram Thioflavin TG (C.l. 49005) 5 ml. methanol Up to about 0.5% by weight of dye with respect to the weight of zinc oxide may be incorporated. However, it is preferred to incorporate the minimum amount of dye capable of producing the desired result. Mixtures vof dyes may also be used, for example methylene gray. or mixtures of other dyes if they are compatible. The above-mentioned dyes are the commercially obtainable dyes listed according to their commercial names. Each 0f these dyes is chemically defined according to its name and C.I. No. (Second Edition). The dyes as supplied may contain more or less amounts of inert ingredients according to the process by which they are made.

The sensitizing dye is believed to be adsorbed on the surfaces of the photoconductive white zinc oxide particles. Radiant energy to which the zinc oxide particles are substantially insensitive incident upon the zinc oxide- -binder layer is absorbed by adsorbed dye molecules. The absorbed energy with more or less eiciency is then transferred to the zinc oxide particle increasing the electrical conductivity thereof.

The recording elements herein therefore exhibit the normal photoconductive spectral response of white zinc oxide-binder recording elements and also an additional photoconductive spectral response in the visible region lof the spectrum. It may be stated that the dye broadens the light absorption band of the zinc oxide. This spectral response is predetermined by the selection of the particular sensitizing dye. The energy transfer is not believed to be thermal because the spectral sensitivity due to the dye is orders of magnitude faster and more efcient than thermal processes.

The sensitizing eect of a dye upon photoconductive white zinc oxide depends on several factors. Some importantfactors are (1) the amount of light absorbed by the dye, (2) the absorption characteristics of the dye in the light spectrum, and (3) the efficiency of the transfer of energy absorbed. to the photosensitive zinc oxidebinder element will play a part. Two dyes may have very similar light absorption characteristics and one be a poor sensitizer while the other with better energy transfer be a good sensitizer. Some dyes have no appreciable sensitizing action for the zinc oxide-binder element at all.

The coating mix of Example l may be prepared in several ways. The simplest method is to dissolve the film-forming material in an organic solvent capable of effecting solution and then dispersing the other ingredients therein. Alternatively, the ingredients may be dry-blended by kneading with a film-forming binder added at a sufiiciently high temperature to render it plastic. In the case of waxes, the ingredients may be mixed together in the molten wax.

The proportion of photoconductive white zinc oxide to electrically-insulating, lm-forming binder in the finished coating may be varied. 'I'he preferred ranges are 50 to 90% of zinc oxide and 50 to 10% of nlm-forming binder. The optimum proportions are dependent upon the nature of the photoconductor, the film-forming binder, the dye and the results desired.

EXAMPLE 8 Vmixture is obtained. The dyed zinc oxide is filtered off and dried. A coating mix comprising 120 grams of dyed zinc oxide, 80 grams of a 60% silicone resin solution and 106 grams of toluol is ball-milled and coated on a sheet of paper as in Example 1. .The materials of this example may be the same materials as in Example 1 with the exception of the dye.

EXAMPLE 9 An improved electrophotographic recording element may also be prepared by first coating a backing member `with a zinc oxide-binder mix and then dyeing the coating with a suitable dye. For example, prepare a recording element according to Example l except exclude the dye. Prepare a dye solution comprising 0.05 gram of erythrosin in 200 cc. of ethyl alcohol. Moisten the surface of the zinc oxide-binder coating of the recording element With the dye solution. For example, immerse the coating in the solution or apply a swab of cotton moistened with the dye solution. The recording element is dried and is ready for use.

The recording elements of the invention may be utilized to receive a visible image thereon. For example, referring to FIGURE 3, the recording element of `Example 1 is placed with its coated surface 23 facing upwards and its backing member 21 against a metal ground plate 51. An electrostatic charging device 61 is passed, in darkness, over the photoconductive coating 23 to provide an over-all electrostatic charge thereon. The charging device 61 may comprise an array of line wires 53 mounted near the grounded metal plate 51. A source of D.-C. high voltage is connected between the wires 53 and the ground plate 51 to provide a negative potential of about 6000 volts on the wires with respect to the ground plate. The voltage is not critical but should be sufficiently high to cause a corona discharge adjacent the wires 53. The surface of the coating 23 of the recording element passing under the charging device 61 becomes charged negatively. The apparatus and process may produce a blanket positive charge if the polarly of the Wires 53 is positive with respect to the ground plate 51.

The next step in the process is to discharge selected areas of the charged surface of the recording element in order to produce an electrostatic image thereon. Referring to FIGURE 4, this may be accomplished by exposing the recording element to a light image derived, for example, from a projector 59 containing the printed matter to be reproduced. The light image is focused on the charged surface of the coating 23. The object to be printed may, however, be any object used in ordinary photographic processes. Wherever the light strikes the surface of the coating 23, the electrostatic charge thereon is reduced or removed. This leaves an electrostatic image or pattern of electrostatic charges corresponding to the dark portions of the light image. Other methods of producing an electrostatic image may also be used.

The electrostatic image may be stored for a time in darkness if desired. Ordinarily the next step is to develop the electrostatic image. Referring to FIGURE 5, development may be accomplished by maintaining the recording element in darkness and passing a developer brush 5S containing a developer powder (toner) across the surface of the coating 23 bearing the electrostatic image. The developer brush comprises a mixture of magnetic carrier particles, for example, powdered iron and developer powder particles (toner particles). The mixture is secured in a magnetic field by a magnet 57 to form a developer brush. This method of development is more completely described by H. G. Greig in U.S. Patent No. 2,874,063, issued February 17, 1959. The powder image, produced by the development of the electrostatic image, may be fixed directly to the recording element or may be transferred to another surface and fixed thereon. Other methods of development and xing well known in the electrostatic printing art may be used.

If the colored background of the recording element due to the dye sensitization according to the invention is objectionable, the dye may be bleached after the image is developed. One convenient process for bleaching is by exposure to light. For example, kryptocyanine and pinacyanole are bleached by exposure to visible light iu less than a minute.

The rate of bleaching by light depends upon the dye concentration, the intensity and spectral range of illumination and the nature of the dye. Most of the dyes enumerated herein may be bleached by exposure to light. Bleaching by exposure to light may be accomplished in a fraction of a second for some dyes under special circumstances. The incorporated dyes may also be bleached by chemical action by methods known in the chemical art.

Dye sensitization of the photoconductive white zinc oxide-binder electrophotographic elements herein are useful in both black-and-White and in color printing. The utility can be quite different from that normally achieved in silver halide photographic elements.

For example, one may provide a recording element sensitized with a mixture of dyes so it has a panchromatic response but with widely different sensitivities to different spectral bands in the visible light range. See examples above. A recording element of this character can be used to print different colors from a single color transparency by developing the images with non-overprinting toners and by using only a change in exposure time to go from one color to another in a repeat printing procedure. If for example the sensitivity is highest to red light, lower to yellow-green light and lowest to blue light, then with an incandescent light source, an exposure sufficient to discharge the image in the yellow-green areas will also discharge the white and red areas and only the blue image will retain a charge. If this image is developed with a toner that will not retain a charge (a non-overprinting toner) then in a repeat printing procedure with exposure time reduced to just clear the red image areas, only the yellow-green areas will retain a charge. The white areas which receive the most light will be discharged with the red and the previously developed blue image will not retain a charge. In a like manner, the exposure time can be reduced further to just clear the white areas and the red image can be printed. If this is the last step any toner can be used to develop the image. This has no counterpart in silver halide photography.

One may balance a mixture of dyes to sensitize the photoconductive white zinc oxide-binder element so that it can have very nearly equal response panchromatically to light from a given light source. Such an element is useful not only for black-and-white printing but also for color printing from color transparencies and from opaque color prints by reflection optics. In color printing, filters are used to mask the light in the band Where it is desired to print. A repeat charge and exposure is of course needed for each color. With dye sensitization a considerable increase in sensitivity is gained when a wide band emission light source, such as an incandescent lamp, is used.

Printing by reflection optics, especially from a subject having several different colors where the reilection of ultraviolet light from a background and a colored image may be very nearly the same, is made possible by the dye sensitization herein. The spectral response of the photosensitive element can be made to approximate the response of the human eye in the visible spectrum so that even a black-and-white print from a colored subject can show the color intensities in the proper relationship.

By the dye sensitization herein, one may match the light from a mono-chromatic or a narrow emission band light source such as a cathode ray tube. If such light is entirely in the visible range, the white zinc oxide-binder recording element without dye sensitization would be substantially insensitive. For example, fluorescein may be used in a recording element herein to sensitize for the light from a P11 phosphor in a CR tube.

What is claimed is:

l. A recording element for electrostatic photographic printing characterized by exhibiting the photoconductive spectral response of photoconductive White zinc oxidebinder recording elements and also an additional photoconductive spectral response in the Visible region of the spectrum, said recording element including a photoconductive insulating layer consisting essentially of iinelydivided photoconductive white zinc oxide particles in an electrically-insulating, nlm-forming binder and up to about 0.5% by Weight with respect to the weight of said zinc oxide particles of at least one optically sensitizing dye which extends the photoconductive spectral sensitivity of said zinc oxide in the visible region of the spectrum, said zinc oxide particles constituting 50% to 90% by weight of said layer and said binder constituting substantially all of the balance of said layer.

2. A recording element for electrostatic photographic printing comprising a backing member carrying a photoconductive insulating layer about 0.0001 to 0.0015 inch thick consisting essentially of finely-divided photoconductive white zinc oxide particles dispersed in an electricallyinsulating, nlm-forming binder and up to about 0.5% by weight with respect to the weight of said zinc oxide particles of at least one optically sensitizing dye which extends the photoconductive spectral sensitivity of said zinc oxide in the visible region of the spectrum, said zinc oxide particles constituting 50% to 90% by weight of said layer and said binder constituting substantially all of the balance of said layer.

3. A recording element for electrostatic photographic printing comprising a backing member having a surface coated with a mixture consisting essentially of finely- 10 divided photoconductive white zinc oxide particles dispersed in an electrically-insulating, hlm-forming binder and up to about 0.5% by weight with respect to the weight of said zinc oxide particles of at least one optically sensitizing dye which extends the photoconductive sensitivity of said zinc oxide in the visible region of the spectrum, said zinc oxide particles constituting 50% to 90% by weight of said mixture and said binder constituting substantially all of the balance of said mixture.

4. A recording element for electrostatic photographic printing comprising a backing member carrying a layer consisting essentially of finely-divided photoconductive white zinc oxide particles dispersed in an electricallyinsulating, film-forming binder, said zinc oxide particles having adsorbed thereto up to about 0.5% by weight with respect to the weight of said zinc oxide particles of at least one optically sensitizing dye which extends the photoconductive spectral sensitivity of said zinc oxide in the visible region of the spectrum and selected from the group consisting of fluorescein, eosin, phloxine, phloxine B, erythrosin, rose bengal, malachite green, crystal violet, Ibasic fuchsin, methyl green, brilliant green, patent blue, brilliant blue PCF, thioilavine TG, methylene green, neutral red, safranine Y, methylene violet, auramine O, ethyl red, thiazole purple, kryptocyanine, pinacyanole, methylene blue, ecridine orange, methylene gray, alizarin red S, carbanthrene yellow G, and quinizarin, said zinc oxide particles constituting 50% to 90% by Weight of said layer and said binder constituting substantially all of the balance of said layer.

5. The'recording element of claim 4 wherein said dye is rose bengal.

6. The recording element of claim 4 wherein said dye is erythrosin.

7. The recording element of claim 4 wherein said dye is fluorescein.

8. The recording element of claim 4 wherein a plurality of dyes are adsorbed on said zinc oxide particles.

9. The recording element of claim 4 wherein a combination of fluoreceim rose bengal and brilliant green are adsorbed on said zinc oxide particles.

10. The recording element of claim 4 wherein a combination of rose bengal, iiuorescein, auramine O and thioavin TG are adsorbed on said zinc oxide particles.

11. A recording element for electrostatic photographic printing characterized by exhibiting the photoconductive spectral response of photoconductive White zinc oxide and also an additional photoconductive spectral response in the visible region of the spectrum, said element comprising a paper backing member carrying a layer about 0.0001 to 0.10015" thick and having a composition consisting essentially of nely-divided photoconductive White zinc oxide particles in an electrically-insulating silicone resin binder and up to 0.5 weight percent with respect to the weight of said zinc oxide of an optically sensitizing dye which extends the photoconductive spectral sensitivity of said zinc oxide in the visible region of the spectrum, said zinc oxide particles constituting 50% to 90% by weight of said layer and said binder constituting substantially al1 of the balance of said layer.

l2. A recording element for electrostatic photographic printing characterized by exhibiting the photoconductive spectral response of photoconductive White zinc oxide and also an additional photoconductive spectral response in the visible region of the spectrum, said element consisting essentially of a paper backing member carrying a layer about 0.0001 to 0.0015" thick and having a composition consisting essentially of finely-divided photoconductive white zinc oxide particles in an electrically-insulating silicone resin binder and up to 0.5 weight percent with respect to the weight of said zinc oxide of rose bengal, said zinc oxide particles constituting 50% to 90% by weight of said layer and said binder constituting substantially all of the balance of said layer.

13. A recording element for electrostatic photographic printing characterized by exhibiting the photoconductive spectral response of photoconductive white zinc oxide and also an additional photoconductive spectral response in the visible region of the spectrum, said element consisting essentially of a paper backing member carrying a layer about 0.0001 to 0.00115 thick and having a composition consisting essentialy of timely-divided photoconductive White'zinc oxide particles in an electrically-insulating silicone resin binder and up to 0.5 Weight percent with respect to the weight of said zinc oxide of fluorescein, said zinc oxide particles constituting 50% to 90% by Weight of'said layer and said binder constituting substantially all ofthe balance of said layer.

14. A recording element for electrostatic photographic printing characterized by exhibiting the photoconductive spectral response of photoconductive white zinc oxide and also an additional photoconductive spectral response in the visible region of the spectrum, said element consisting essentially of a paper backing member carrying a layer about 0.0001 to 0.0015 thick and having a composition consisting essentially of finely-divided photoconductive White zinc oxide particles in an electrically-insulating silicone resin binder and up to 0.5 Weight percent with vReferences Cited Vin the le of this patent UNITED STATES PATENTS 1,730,505 Hart Oct. 8, 1929 12 1,840,459 Marshall Jan. 12, 1932 2,010,388 Block Aug. 6, 1935 2,169,840 Lewis etal Aug. 15, 1939 2,287,161 Ball June 23, 1942 2,297,691 Carlson v Oct. 6, 1942 2,459,874 Fay Jan. 25, 1949 2,551,582 Carlson May 8, 1951 2,554,017 Dalton May 22, 1951 2,599,542 Carlson June 10, 1952 2,663,636 Middleton Dec. 22, 1953 2,692,178 Grandadam Oct. 19, 1954 2,693,416 Buttereld Nov. 2, 1954 2,727,808 Thomsen Dec. 20, 1955 OTHER REFERENCES Chemical Abstracts, 43, 7349d.

Chemical Abstracts, 45, 5018e.

Putseiko et al.: Translation from Doklady, Akademi Nauk. SSSR, 90, 1005-08, pages 1-5, translation.

Allen: Photo-Electricity, page 75.

Phosphor-Type Photoconductive Coatings for Continuous Tone Electrostatic Electrophotography, 1952, Photographic Engineering, vol. 3 (No. 1), pages 16 and 17 particularly relied on.

Wall: The History of Three-Color Photography, page 211 and note on'page 238, American Photographic Pub. Co., Boston, 1925. 

1. A RECORDING ELEMENT FOR ELECTROSTATIC PHOTOGRAPHIC PRINTING CHARACTERIZED BY INHIBITING THE PHOTOCONDUCTIVE SPECTRAL RESPONSE OF PHOTOCONDUCTIVE WHITE ZINC OXIDEBINDER RECORDING ELEMENTS AND ALSO AN ADDITIONAL PHOTOCONDUCTIVE SPECTRAL RESPONSE IN THE VISIBLE REGION OF THE SPECTRUM, SAID RECORDING ELEMENT INCLUDING A PHOTOCONDUCTIVE INSULATING LAYER CONSISTING ESSENTIALLY OF FINELYDIVIDED PHOTOCONDUCTIVE WHITE ZINC OXIDE PARTICLES IN AN ELECTRICALLY-INSULATING, FILM-FORMING BINDER AND UP TO ABOUT 0.5% BY WEIGHT WITH RESPECT TO THE WEIGHT OF SAID ZINC OXIDE PARTICLES OF AT LEAST ONE OPTICALLY SENSITIZING DYE WHICH EXTENDS THE PHOTOCONDUCTIVE SPECTRAL SENSITIVITY OF SAID ZINC OXIDE IN THE VISIBLE REGION OF THE SPECTRUM, SAID ZINC OXIDE PARTICLES CONSTITUTING 50% TO 90% BY WEIGHT OF SAID LAYER AND SAID BINDER CONSTITUTING SUBSTANTIALLY ALL OF THE BALANCE OF SAID LAYER. 